1. Field of the Invention
This invention relates to a process for the conversion of meta/para-cresol mixtures by catalytically methylating the meta/para-cresol mixtures in their ortho-positions, separating the 2,3,6- and 2,4,6-trimethylphenols formed and converting subsequently the 2,4,6-trimethylphenol in the presence of iron oxide(s) or catalysts which contain iron oxide(s) and at least one additional oxide, to a mixture of methyl phenols, which is essentially free from meta-substituted methylphenols.
2. Description of the Prior Art
The formation of 2,3,6-trimethylphenol by catalytically methylating meta-cresol is known. DE-PS No. 30 12 357 discloses for example the formation of 2,3,6-trimethylphenol in 99.5% selectivity by methylating pure m-cresol in the presence of catalysts, which contain iron oxide.
If pure para-cresol is methylated with methanol or dimethylether according to this state of the art, 2,4,6-trimethylphenol is obtained with 99% selectivity. Numerous other catalysts are known, which lead to highly selective o-substitution of phenols. For example in DE-OS No. 27 16 035 a copper-chromium-oxide catalyst is disclosed, whereas according to DE-AS No. 25 47 309, catalysts consisting of Fe.sub.2 O.sub.3 /SiO.sub.2 /Cr.sub.2 O.sub.3 and Fe.sub.2 O.sub.3 /SiO.sub.2 /Cr.sub.2 O.sub.3 /K.sub.2 O are well suited for the o-methylation of phenols.
2,3,6-trimethylphenol is an important starting material for the production of 2,3,6-trimethylhydroquinons, which is a component of Vitamin E.
One of the technically applied processes for the production of 2,3,6-trimethylphenol is based on open-chain units, which are condensed to a six-membered ring, which is subsequently dehydrated to form a substituted phenolring. This process is disclosed in a number of patents, among others in DE-AS No. 16 68 874 and DE-PS No. 17 93 037.
If, according to the state of the art, easily accessible mixtures of meta- and para-cresol are methylated, besides the desired 2,3,6-trimethylphenol the undesirable 2,4,6-trimethylphenol is obtained in amounts, which depend on the amount of p-cresol in the mixture.
Selective isomerizations of 2,4,6-trimethylphenol in the presence of phenol and/or phenol derivatives are disclosed in DE-PS No. 33 04 663 (U.S. patent application, Ser. No. 577,849) which has been filed by applicant. In this process a product mixture is obtained, which is essentially free of meta-substituted alkylphenols.
According to the above named U.S. patent application applicant has found that iron oxide(s) alone as well as numerous combinations of iron oxide(s) with other oxides are excellently suited for the selective isomerization and transalkylation of 2,4,6-trimethylphenol.